The Sixth Extinction: An Unnatural History

corals and tabulate corals were only distantly related to today’s
scleractinian corals, and both orders died out in the great extinction at
the end of the Permian. This extinction shows up in the geologic record as
(among other things) a “reef gap”—a period of about ten million years
when reefs went missing altogether. Reef gaps also occurred after the late
Devonian and the late Triassic extinctions, and in each of these cases it
also took millions of years for reef construction to resume. This
correlation has prompted some scientists to argue that reef building as an
enterprise must be particularly vulnerable to environmental change—yet
another paradox, since reef building is also one of the oldest enterprises
on earth.
Ocean acidification is, of course, not the only threat reefs are under.
Indeed, in some parts of the world, reefs probably will not last long
enough for ocean acidification to finish them off. The roster of perils
includes, but is not limited to: overfishing, which promotes the growth of
algae that compete with corals; agricultural runoff, which also
encourages algae growth; deforestation, which leads to siltation and
reduces water clarity; and dynamite fishing, whose destructive potential
would seem to be self-explanatory. All of these stresses make corals
susceptible to pathogens. White-band disease is a bacterial infection that,
as the name suggests, produces a band of white necrotic tissue. It afflicts
two species of Caribbean coral, Acropora palmata (commonly known as
elkhorn coral) and Acropora cervicornis (staghorn coral), which until
recently were the dominant reef builders in the region. The disease has so
ravaged the two species that both are now listed as “critically
endangered” by the International Union for Conservation of Nature.
Meanwhile coral cover in the Caribbean has in recent decades declined by
close to eighty percent.
Finally and perhaps most significant on the list of hazards is climate
change—ocean acidification’s equally evil twin.
Tropical reefs need warmth, but when water temperatures rise too
high, trouble ensues. The reasons for this have to do with the fact that